Manganese dioxide is known and widely used as a solid electrolyte in electrolyte capacitors. Such capacitors are conventionally formed by first anodizing a valve-metal anode (e.g., tantalum) to form a dielectric oxide coating, and thereafter immersing the oxide-coated anode in an aqueous solution of manganese nitrate. After a sufficient period of time, the wet anode is heated to cause pyrolytic decomposition of the manganese nitrate to manganese dioxide. To achieve the desired thickness of the solid electrolyte, the steps of immersion and heating are often repeated multiple times. Unfortunately, one problem with conventional manganizing techniques is that the thickness of the resulting manganese dioxide is often greater at certain locations of the anode (e.g., edges), which can lead to poor electrical performance. Various techniques have been employed in an attempt to address these problems. For example, surfactants have been employed in the manganese nitrate solution to substantially reduce its surface tension and improve the wettability of the surface of the oxide-coated anode. One such surfactant is Erktantol® NR (Tanatex Chemicals BV), which is a nonionic fatty alcohol polyglycol ether. Likewise, U.S. Pat. No. 4,302,301 to Tierman describes various other nonionic surfactants that can be employed in the manganizing solution, such as nonylphenoxypoly-(ethyleneoxy)ethanol (Igepal CO-630); isooctylphenoxy-polyethoxyethanol (Triton X-100), benzyletheroctylphenol-ethylene oxide condensate (Triton CF-10), and 3,6-dimethyl-4-octyne-3,6-diol (Surfynol 82).
Although the addition of surfactants may provide some benefits, significant problems nevertheless remain. For example, high voltage power distribution systems deliver a high voltage to the capacitor that can result in an inrush or “surge” current, particularly during a fast switch on or during an operational current spike. The peak surge current that the capacitor can withstand without failure is believed to be in part related to the quality of the manganese dioxide layer. More particularly, a poor quality layer may contain peaks and valleys in which thinner areas are located adjacent to thicker areas. Because the thinner areas have a lower resistance than the thicker neighboring areas, the power dissipated in the thinner areas is generally greater. Therefore, when a surge current is applied, these thinner areas may develop into weak “hot spots” that ultimately lead to degradation and breakdown of the dielectric. In addition to the issues noted above, conventional capacitors may also exhibit other issues, such as a relatively large loss in capacitance when wet and a high leakage current.
As such, a need currently exists for an improved electrolytic capacitor containing a manganese oxide solid electrolyte.